The goal of the proposed research is to study the basis of T cell-mediated immune recognition of xenogeneic cellular grafts, using islets of Langerhans as a relevant model system. Data obtained during the funding period strongly suggests that the prevalent mode of xenogenic islet recognition is through the indirect pathway of antigen recognition by recipient's CD4+ T lymphocytes. In Specific Aim I a, several immunodeficient mice will be used to determine the mechanisms of CD4+ T lymphocyte-dependent xenoreactivity towards rat islets. The notion that islet xenografts survive in nude and SCID recipients strongly suggests that, it least in murine recipients, rejection of such grafts is dependent on T cells. Furthermore, evidence was obtained during the first funding period that such cellular response is CD4+ T cell-dependent in vivo. The first goal of this proposal is to analyze the role of molecules involved in target cell destruction in xenograft rejection via direct lysis (perforin), via induction of apoptosis (Fas L) and by mediation of inflammation (IFN-g). Perforin deficient mice (PKO), Fas L deficient mice (gld/gld) and IFN-g deficient mice will be used as donors of CD4+ T cells, which will be transferred into RAG1-/- recipients. RAG 1-/- (T and B cell deficient, not as leaky as SCID mice) recipients will be previously transplanted with rat islets. The hypothesis tested by this series of experiments is that CD4 T cell-mediated rejection of xenogeneic islets requires triggering of inflammation, but it does not require the presence of intact cell-cytotoxicity mediating molecules. Therefore the expected results (supported by preliminary findings) are that PKO CD4 cells will induce rat islet rejection in times similar to those of WT CD4 cells, while IFN-g-deficient CD4 cells will be significantly less efficient in mediating graft rejection. While the pivotal role of CD4+ T cells in xenogeneic islet rejection has been established, it can not be formally ruled out that non T cell-mediated effector mechanisms might contribute (partially or entirely) to the observed phenomena. Specific Aim I b is therefore set forth to explore this working hypothesis. Specific Aim I c will test the hypothesis that CD4+ T cell-mediated rejection of xenogeneic grafts might not require T cell receptor-mediated interaction with the grafted target cells. Specific Aim I d is focused on the analysis of rejection of discordant islet grafts in the pig to mouse model system. In Specific Aim II, the P.I. proposes to investigate the mechanisms that mediate the induction of long term survival of xenogeneic islets grafted in mice treated with a short course of anti-LFA-1 antibodies.